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Clearly it hasn’t been commodities’ week, month or year. But focusing on the worst-performers doesn’t tell the full story.

The Bloomberg Commodity Index fell 7.2 percent this month, touching a 13-year low on June 20. The biggest losers are crops such as coffee and sugar, each down 27 percent. They’ve fallen because of a weak currency in the most important producer, Brazil, and abundant inventories after years of surpluses, according to Commerzbank. (They’re also very weather-driven.)

But those aren’t actually the reasons driving down the whole index. The biggest contributor to the index’s decline has been industrial metals such as copper and nickel, down 12 percent and 23 percent this year, respectively. They’re falling because of concerns that the tumbling Chinese stock market betrays sapping demand from the world’s biggest consumer.

What’s behind the discrepancy? More valuable commodities like oil and gold (down 12 percent and 6.8 percent) have heavier weightings in the index (and most exchange-traded products and other portfolios). As it happens, there’s a lot of overlap between the smallest members and the biggest nominal declines.

To contact the author on this story: Isaac Arnsdorf at iarnsdorf@bloomberg.net

To contact the editor on this story: David Marino at dmarino4@bloomberg.net

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